A screw compressor is widely applied in the field of refrigeration air conditioners due to its wide application scope and high reliability. According to application requirements, a single screw compressor often runs under various working conditions, for instance cold water and ice making working conditions of an ice-storage dual-working-condition unit, or cold water and hot water making working conditions of a heat pump unit, or even triple-working-conditions required to make ice, cold water and hot water.
Different application working conditions require different compressor internal volume ratios Vi, wherein: Vi=Vs/Vd, Vs refers to an intertooth volume when the screw compressor finishes suction and starts compression; Vd refers to an intertooth volume when the screw compressor finishes compression. It is well-known when design Vi of the screw compressor is equal to system Vi′, the screw compressor provides optimal performance and operation state, and at this moment, vibration level of the screw compressor is comparatively low ensuring its reliability. Wherein, system Vi′=(Pd/Ps)^(1/k); Pd refers to pressure at an exhaust end of the compressor; Ps refers to pressure at an suction end of the compressor; k refers to adiabatic exponent of compression media.
According to the design types of Vi, a screw compressor can be screw compressor with fixed Vi or screw compressor with adjustable Vi.
With respect to a screw compressor with fixed Vi, FIG. 1 and FIG. 2 illustrate the diagrammatic sketches of the screw compressor with fixed Vi respectively corresponding to low Vi design and high Vi design. It can be seen from the sketches that with adjustment of length of a slide valve 3, various requirements of design Vi are able to be achieved. For such a type of screw compressor, the main function of the slide valve 3 is to adjust efficient air volume, and Vi of the screw compressor is basically fixed and automatic matching of system Vi′ is not possible. It is designed aiming at only one working condition, and when it runs under the design working condition, Vi=Vi′, the efficiency of the compressor is supreme. In addition, according to various application working conditions, it is needed to select different design Vi of the screw compressor, for instance under cold water making working condition, design Vi of the screw compressor is usually 1.7˜2.6, under heating or ice-storage working condition, design Vi of the screw compressor is usually 3.0˜4.0 and for refrigeration or high temperature heat pump application, design Vi of the screw compressor reaches 3.5˜5.0. Taking a screw compressor with R134a heat pump for example, under cooling working condition, the temperature of yielding water of its evaporator is 7° C. and of its condenser is 35° C.; under heating working condition, the temperature of yielding water of its evaporator is 7° C. and of its condenser is 55° C. Calculation of system Vi: under cooling working condition, Vi′=2.3; under heating working condition Vi′=3.5. Therefore, for fixed Vi compressor selection, it is only able to select a single design Vi between 2.3 or 3.5, and is not in a position to satisfy the requirements of system Vi′ for both cooling and heating conditions; if design Vi of the compressor is selected as 2.3, the level of vibration of the compressor under heating working condition will be too high, affecting service life of the compressor; if design Vi of the compressor is selected as 3.5, COP of the compressor under cooling working condition will decay 10%˜15% compared with that of the compressor with 2.3 design Vi.
With respect to a screw compressor with adjustable Vi, FIG. 3 illustrates a common screw compressor with adjustable Vi at present, which adds a Vi adjusting block 4 on the basis of a slide valve 3; with the action of the Vi adjusting block 4, adjustment of Vi is realized, and thus matching between Vi of compressor and system Vi′ is achieved, which enables high efficient operation of the compressor. However, the limitation of the compressor with such a design is: due to the additional Vi adjusting block 4, the structure of the compressor becomes complicated and the manufacture costs are increased, which blocks its popularization in air conditioner system (cooling/heating/ice-storage) application; when the compressor runs with high Vi, since the Vi adjusting block 4 occupies most of room of a screw rotor, with increase of Vi, capacity adjustment ability of the slide valve 3 of the compressor is obviously decreased, resulting in the issue of discontinuous adjustment over loads and poor ability of adjustment over a part of loads. When the compressor runs with high Vi, the adjustable region by the slide valve is shortened, ability of adjustment over a part of loads becomes poor, and discontinuous adjustment of capacity occurs, for instance at 100%˜60% load region, the slide valve is not able to do adjustment, and the higher Vi is, the poorer capacity adjustment ability will be.